This invention relates to an antenna unit used as a GPS (Global Positioning System) antenna.
As well known in the art, a GPS receiver is an apparatus for detecting a current position of a mobile station for a user by receiving electric waves radiated on an earth from a plurality of GPS (Global Positioning System) satellites which go over the earth.
As well known in the art, the GPS (Global Positioning System) is a satellite positioning system using military satellites under Department of Defense in Unite States control that comprise twenty-four non-geostationary satellites in total in six orbit surfaces every four satellites at an orbit height of about 20,000 km. The above-mentioned non-geostationary satellites (military satellites) are called GPS satellites. If the GPS receiver receives electric waves from four GPS satellites, it is possible to carry out a three-dimensional positioning. In this connection, if the GPS receiver receives electric waves from three GPS satellites, it is possible to carry out a two-dimensional positioning.
In other words, the GPS is a global positioning system comprising twenty-four artificial satellites launched by Department of Defense in United States, a control station on earth, and mobile stations for users. By using the global positioning system, it is possible to calculate a position, a moving direction, and a moving speed of the mobile station by measuring distances between the mobile station and three or more GPS satellites on the basis of time intervals taken for arrival of the electric waves. Although the global positioning system is originally used for military affairs, presently, it is widely applied to car navigation systems or the like. In addition, the mobile stations may be not only automobiles but also airplanes, ships, or the like.
Now, xe2x80x9ccar navigationxe2x80x9d means to provide a driver information by displaying a position of a driver""s driving car on a map of a car mounted machine at a real time, by displaying road traffic information, and by calculating the most suitable route up to a driver""s destination.
A current used car navigation system calculates a latitude, a longitude, a height, and a time instant on capturing four or more GPS satellites and calculates the latitude, the longitude, and the time instant with the height fixed on capturing only three GPS satellites. In addition, the current used car navigation system calculates the latitude and the longitude using a time instant of an internal clock with the height fixed on capturing only two GPS satellites. Furthermore, the current used car navigation system carries out an error indication on capturing only one GPS satellite or no GPS satellite (see Japanese Unexamined Patent Publication Tokkai No. Hei 9-236650 or JP-A 9-236650.
Now, inasmuch as an electric wave called a GPS signal, which is generated by the GPS satellite and is arrived on the ground, has a very weak strength, the GPS signal may be buried in or covered with noises of electric waves on the ground. Accordingly, as the GPS signal, a PSK (Phase Shift Keying) wave which spread spectrum modulated by using a PN (Pseudo Noise) code is used and the GPS receiver comprises a LNA (Low Noise Amplifier) circuit for extracting the GPS signal from the noises and for amplifying an extracted GPS signal.
Attention will be directed to the car navigation system where the mobile station is a car or an automobile. In this event, a GPS antenna (or an antenna unit) is mounted on an outer surface of a body of the car by using magnets or the like. Specifically, it will be assumed that the GPS antenna (or the antenna unit) is a planer-type antenna. The planer-type antenna may be mounted on a metallic roof panel of the car or the like by magnetically attracting the planer-type antenna to the metallic roof panel. The GPS antenna (or the antenna unit) comprises an antenna element and a circuit board on which accompanied circuit elements including the above-mentioned LNA circuit are mounted. The planer-type antenna is called a patch antenna in the art.
In the manner which will later be described in conjunction with FIG. 3, the planer-type antenna has almost no gain (directivity) in a horizontal direction. Accordingly, it is difficult or unsuitable to mount the planer-type antenna on an inclined place.
It is therefore an object of the present invention to provide an antenna unit which is capable of mounting the antenna unit on not only a horizontal place but also an inclined place such as a rear window or a front window of a car.
Other objects of this invention will become clear as the description proceeds.
According to a first aspect of this invention, an antenna unit comprises a circuit board on which circuit elements are mounted. The circuit board has a principal surface. Mounted on the principal surface of the circuit board, an antenna element is connected to the circuit elements. The antenna element comprises a helical antenna having an axial direction which extends in a direction in substantially parallel to the principal surface of said circuit board.
According to a second aspect of this invention, an antenna unit comprises a circuit board having a principal surface and a back surface opposite to the principal surface. Circuit elements are arranged on the back surface of the circuit board. Mounted on the principal surface of the circuit board, an antenna element is connected to the circuit elements. The antenna element comprises a helical antenna having an axial direction which extends in a direction in substantially parallel to the principal surface of the circuit board. Added to the back surface of the circuit board so as to cover the circuit elements, a shield cover shields the circuit elements.
According to a third aspect of this invention, an antenna unit comprises a circuit board having a principal surface and a back surface opposite to the principal surface. Circuit elements are arranged on the back surface of the circuit board. Mounted on the principal surface of the circuit board, an antenna element is connected to the circuit elements. The antenna element comprises a helical antenna having an axial direction which extends in a direction in substantially parallel to the principal surface of the circuit board. Added to the back surface of the circuit board so as to cover the circuit elements, a shield cover shields the circuit elements. An output cable is connected to the circuit elements inside the shield cover. The output cable is pulled out of the shield cover.